Known tape measures include tape measures having a measuring blade which is spooled within a casing. The blade is extendable from the casing by pulling its free end. When released, the blade is automatically retracted back into the casing. Typically, this retraction is due to a spring within the casing, for example a spring connected between the casing and a reel on which the blade spools.
For ease of manufacture, the casing is typically made from moulded plastics material. This allows the casing to be relatively lightweight and easy to handle. The moulding operation also allows the incorporation of detailed functional shapes into the casing in a single forming step.
Usually, the blade of the measure is metallic. For many applications, the blade must be strong and inextensible. Furthermore, it should also be stiff. This is particularly the case where the blade should be able to extend from the blade and hold itself in an approximately straight configuration without support at its distal end (the end furthest from the casing). Often, this desirable feature is achieved using a relatively thin blade which, when pulled longitudinally straight from the casing, has a transversely curved character. It is this transverse curvature which acts to maintain the blade in an approximately straight longitudinal configuration.
Typically, other materials (e.g. most plastics materials) do not give the blade the above advantageous features. In particular, most plastics materials are not stiff enough to enable the blade to remain approximately straight when extended from the casing and yet flexible enough to allow the blade to be spooled easily within the casing and tough enough to withstand repeated use.
However, one disadvantage of metallic blades is their weight. Most suitable metallic materials for the blade are more dense than equivalent plastics materials which might be considered for the application. For this reason, the blade is usually by far the heaviest (i.e. most massive) part of the tape measure.
Associated with the blade is the spring which acts to recoil the blade back into the casing after the blade has been extended from the casing. Again, the spring is usually made from metal and so is dense and therefore heavy.
The blade is usually spooled, within the casing, on a spool or other rotatable mounting. The blade is therefore rotatably mounted within the casing, for example a reel or spool may be rotatably mounted on a post within the casing in order to define an axis of rotation for the reel or spool. Typically, the spring is attached between the post and the blade, so that when the blade is unspooled, the spring is tightened to provide a return force on the blade.
Tape measures are often subjected to rough handling. In particular, in building, maintenance or DIY applications, tape measures are often dropped, e.g. from ladders. Drops onto relatively hard surfaces can be catastrophic for the tape measure. This is usually due to a combination of the plastics casing and the heavy blade. Since most of the mass of the whole tape measure is concentrated in the blade, during an impact on the casing, the blade is likely to move relative to the casing. Often, the result of this is damage to the case. More specifically, the post on which the blade is rotatably mounted usually either breaks or breaks away from the casing. This renders the tape measure inoperative or seriously impaired.